The present invention relates to a closed cavity piston assembly for reciprocation in the rotating cylinder block of a hydrostatic unit, such as a pump or a motor. As a result of the unique geometry of the piston assembly components, they can be formed with cost-effective processes involving little or no machining, such as metal injection molding.
Solid steel pistons are well known and have been utilized in the rotatable cylinder blocks of hydrostatic power units. Solid steel pistons are durable, reliable, and relatively inexpensive to make. However, their weight tends to impose limitations on the speed of operation for the cylinder blocks in which they are used. They also develop more operational frictional forces.
Consequently, some manufacturers have searched for cost-effective methods of producing hollow pistons. Open-ended hollow pistons have been produced that weigh less than their solid pistons counterparts, but these hollow pistons do not reduce the compressed oil volume of the hydrostatic unit. Therefore, some manufacturers have welded a cap over the open end of the piston body to form a closed internal cavity in the piston. This reduces the compressible oil volume of the hydrostatic unit. Reduced compressible oil volume is desirable because it provides better control of swashplate moments and better efficiency in swashplate controlled hydrostatic power units.
Although closed cavity hollow pistons can reduce compressed oil volume, they also present some unique problems of their own. Fluid for lubrication and balance is desired at the running surface of a slipper pivotally attached to the piston. One common way to supply such fluid is via a small fluid passageway extending longitudinally through the center of the piston and registering with a similar passage in the slipper. Because introducing fluid into the interior cavity of the hollow piston for this purpose would defeat the purpose of reducing the compressible oil volume, the interior cavity is generally filled with a material such as plastic that is lighter and less compressible than oil. Then the filled piston is drilled to provide the small, centrally located fluid passageway. The filling and drilling operations significantly increase the cost of the hollow pistons and therefore the cost of the hydrostatic units in which multiples of the pistons are used. It is also difficult to get reliable material that can endure the harsh environment of the pistons. Thus, deterioration of the plastic material is a common problem. Aluminum slugs, which are more durable than plastic, have been tried, but they are more difficult to retain within the cavity.
Some pistons include at least three separate and distinct components: a piston body, a piston cap and a hollow tube. The tube is positioned within the cavity and attached to the piston body by one or more washers that extend radially between the outer wall of the tube and the inner wall of the piston body. While pistons of this design solve at least some of the problems outlined above, they are very expensive to make.
Therefore, a primary objective of this invention is the provision of a piston assembly that is cost-effective to produce and reduces the number of finishing operations.
Another objective of this invention is the provision of a hollow or reduced volume piston assembly whose components can be fabricated using metal injection molding techniques then sealingly joined together.
Another objective of this invention is the provision of an economical closed cavity hollow piston with a centrally located fluid passageway extending longitudinal therethrough that is isolated from the rest of the interior cavity.
Another objective of this invention is the provision of a closed cavity piston that has a hollow stem fixed inside the cavity without annular washers attaching it to the wall.
Another objective of this invention is the provision of a closed cavity hollow piston having only two components, a piston body and a piston cap, one of which includes a centrally located hollow stem integrally formed therewith.
Another objective of this invention is the provision of a piston assembly that is easy to assembly due to complementary alignment features on the cap and piston body.
Another objective of this invention is the provision of a hydraulic piston assembly that is quiet and efficient in operation, as well as being capable of being operated in a cylinder block that is rotated at high speeds.
Another objective of this invention is the provision of a piston assembly that is durable and reliable in use.
These and other objectives will be apparent to one skilled in the art from the drawings, as well as from the following description and the claims.